1. Field of the Invention
The present invention relates to a tool holder including: a collet chuck main body having a tapered hole formed in its tip end portion; a double tapered collet having two tapered surfaces, which are a main tapered surface and a tip end tapered surface; and a cylindrical lock nut that is screwed onto external threads formed on the outer periphery of the tip end portion of the collet chuck main body, and thus, presses the taper collet into the tapered hole, thereby reducing the diameter of a tool insertion hole.
2. Description of the Background Art
In a generally known type of tool holders for holding a shank portion of a cutting tool, a lock nut is screwed onto a collet chuck main body, whereby the lock nut presses a tapered collet into a tapered hole of the collet chuck main body. At this time, the diameter of the tapered collet is reduced, whereby the tapered collet holds the shank portion. Various such tool holders have been used in practical applications.
A problem of this type of tool holders is that the tapered collet is tilted with respect to the axis of the collet chuck main body when pressed into the tapered hole of the collet chuck main body. This reduces the deflection accuracy of the cutting tool because the shank portion of the cutting tool is not aligned with the axis of the collet chuck main body.
A collet chuck described in Japanese Utility Model Publication No. H07-27712 of unexamined applications is known as a solution to this problem. In the collet chuck of Japanese Utility Model Publication No. H07-27712 of unexamined applications, a tapered collet is a double tapered collet that includes a tapered surface that is tapered toward the tip end, in addition to a main tapered surface that is closely fitted in a collet chuck main body. Moreover, a lock nut has a lock nut main body, and an annular nose ring that is fitted in the front part of the lock nut main body in a relatively rotatable manner with spherical balls interposed therebetween. An annular stepped portion is formed in the outer periphery of the nose ring so that the outer diameter of the front part of the nose ring becomes smaller than the rear part thereof. The inner diameter of the front end of the lock nut main body is smaller than the outer diameter of the rear part of the nose ring. The annular stepped portion formed in the outer periphery of the nose ring serves as an inner raceway on which the plurality of balls roll. An annular groove is formed in the inner periphery of the lock nut main body, and this annular groove serves as an outer raceway on which the plurality of balls roll.
However, the above conventional tapered collet has the following problem. When fastened with a large force, the lock nut moves rearward toward the collet chuck main body. Thus, the balls are compressed between the outer raceway of the lock nut and the inner raceway of the nose ring, which hinders smooth radial movement of the nose ring for aligning the axis. As a result, the axis of the nose ring is not stably aligned with the axis of the collet chuck main body. Thus, the tapered collet can be tilted with respect to the axis of the collet chuck main body when rotating and fastening the lock nut, and the axis of the tool may not be aligned with the axis of the collet chuck main body, resulting in unstable deflection accuracy of the tool.